This invention relates to a sheet accumulator, and to successive printing presses combined with an intermediate sheet accumulator to produce printed sheets of two or more colors.
The use of two color printing presses, such as stencil screen presses, in sequence to produce two color prints is old. Each press is a functional stage of operation. Each press typically has a dryer, e.g., a UV dryer, to rapidly dry the printed ink from that press before the next functional stage is encountered. To do this, the presses have been known to be put in synchronous drive with each other as with a jack shaft. Unfortunately, the actual processing of the sheets in the first press might get out of synchronism with the processing in the second press for various reasons. For example, if the first press runs out of sheet stock, it has to be stopped temporarily to be reloaded. To continue to run it would cause ink to pass through the stencil screen and be deposited on the equipment. If the first press is stopped and the two presses are synchronously interlocked, the second press will stop. However, since the second press will likely have stock in its dryer, the dryer will quickly overheat the sheet stock to potentially cause a fire. If the second press is caused to continue running, as to prevent stock overheating, it will soon run out of stock from the first press and deposit ink on the equipment. Alternatively, if the second press needs to be stopped for some operational reason, the continued output of sheet stock from the first press will quickly jam up at the discharge end of the first press, causing loss of product and other significant problems. If the first press is stopped simultaneously with stoppage of the second press, the sheet product of the first press will be stalled in the dryer, creating an overheating and potential fire problem. Furthermore, if there are three or more colors to be applied to each sheet, the potential problems of trying to correlate the presses, or the potential costs of operating them independently, become even more magnified.
Consequently, sequential two color sheet printing presses have caused significant operational problems for those in the industry. In view of these problems, it is normally preferred to run each press and dryer independently, even though this requires performing each color printing as a separate operation and greatly increases the cost of printing.